A dominant opening 40 minutes, which was aided by Maama Vaipulu's trip to the sin bin, saw Montpellier pile on 21 points.

Nicolaas Janse van Rensburg and Bismarck du Plessis both crossed, while Ruan Pienaar kicked eight points and Francois Steyn converted a drop goal.

There was no let up after the break as Montpellier added another three tries, while all Castres could muster was Armand Batlle's converted score.

Toulouse is hot on Montpellier's heels thanks to a 37-33 victory over Stade Francais, the accuracy of Zack Holmes and Jean-Marc Doussain proving crucial as both sides scored four tries.

Wandile Mjekevu and Cheslin Kolbe plus 10 points from Holmes gave Toulouse a seven-point advantage at half-time, and although Stade Francais fought back the visitors were able to cling on.

The four points moves them into second place above Racing 92, which faces Lyon on Monday (AEDT).

La Rochelle made it back-to-back wins to cement its top-six place as Bordeaux-Begles was brushed aside 31-20, the host running in five tries.

It is joined in the European places by Pau after a 33-12 triumph over Oyonnax, which slips to the bottom of the table.

Oyonnax drop to 14th place after Brive claimed a first league win of 2018 by beating Agen 15-12.

Gaetan Germain was Brive's hero with five penalties, while Agen could only register four in a tactical battle that also saw three yellow cards.